NEW YORK (CBSNewYork) — Parents are on high alert in New Rochelle after police said a family received a disturbing letter threatening to harm their elementary school-aged children.

As CBS 2’s Andrea Grymes reported, the sexually-explicit letter prompted officials at Jefferson Elementary School to send out a districtwide alert to parents.

“I thought it was sick. I think that person is really sick,” said parent Darlene Rauda.

New Rochelle Parents On Alert Following 'Disturbing' Letter Sent To Family

“I’m really worried right now,” another parent said.

Police said a family received the letter on Tuesday with the return address as Jefferson Elementary. They said the letter included pictures and appeared to threaten the family’s children.

“Sexually explicit and disturbing photographs of two young children. There was also some comments on the photos implying the children might be abducted,” said New Rochelle Police Capt. Joseph Schaller.

New Rochelle police said the letter appears to be an isolated incident, so far. But the mail did spark the school district to robo-call parents, send out alerts, and put a message on its website warning parents to remain vigilant, Grymes reported.

The message posted by the interim superintendent also mentioned the handwritten return address, saying “It was intended to indicate that the letter was an official school mailing. Clearly, it was not.”

Some parents said they’re not so sure.

“Honestly, my personal opinion, I think it might be someone from inside the school. Who else would try get the address of this parent? Makes you wonder,” said Rauda.

Police told Grymes that someone has been sending sexually explicit letters to people in the area for years. They said the people appear to be selected at random.

Investigators, however, do not believe the latest letter is connected to those.

In response police have stepped up their security around the school.

“I see more security around the school and more police officers,” said parent Marisol Valencia.

Investigators urged anyone who noticed or received something suspicious to contact them immediately.